


Random Writing Event - NSFW

by SerifSinclair



Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell, Alternate Universe - Underswap, Clean nonfatal vore, Dominance, Flirting, Fluff, Foncest, Grillcest - Freeform, Incest, M/M, Multi, Okay now we can get to the tags, Other, Overprotective, Spitroasting, Threesome, Underfell Grillby, Underfell Papyrus, Underfell Sans, Underswap Papyrus, Underswap Sans, Vore
Language: English
Status: In-Progress
Published: 2018-05-02
Updated: 2018-05-02
Packaged: 2019-05-01 00:43:49
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 7
Words: 3,717
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14508759
Author URL: https://archiveofourown.org/users/SerifSinclair/pseuds/SerifSinclair
Summary: 20 different prompts submitted through my tumblr, with a large variety of pairings and situations!All are fluffy, consensual, and slow moving ^^; The first chapter contains a list of prompts and pairings for easy navigation for those who want to see something specific~"Papyrus shivered as he followed that thought down a widening road, something that he’d always known but had become more obvious in the last few weeks. He loved his brothers so much, looked up to them, admired them, wanted to be with them forever... Wanted to belong to them, wanted them to fuck him, wanted them to use him and cuddle him and take care of him... and he wondered if they wanted the same thing." - Chapter 3 Excerpt"Sans snarled, and Papyrus finally gave up. Sans felt his brother go limp, arms unfolding and coming to rest back at his sides. Papyrus’ pointed head tipped back until it rested against the wall, throat bared under Sans hand. Good. Sans was the one laying down the rules here; Boss could have his control back once he learned to stop risking his life." - Chapter 5 Excerpt





	1. Navigation Page

**Notes for the Chapter:**

> Not everyone is interested in everything, or they might want to jump to a specific scene, so I hope this table of contents helps!

_**Sort by prompt:** _

1\. List of pairings (You are here)

2\. Fell grill by and ut grillby. Fell is a huge flirt to UT. Setting and details : the surface... That's it.  
-Light me on Fire - Grillby/Fellby - 342 Words

3\. Older brothers underswap sans and his big twin / underswap paps. The big bros are protective of papyrus and realize they maybe are overprotective for a bad reason~  
-Give the World - Blue/Stretch/Blue - 723 Words

4\. Fellby tries to convince grillby to let him tap that... By being lewd and romantic. Will he succeed?? And he manages to get his patrons to help too?  
-Liquid Fire - Grillby/Fellby - 340 Words

5\. Red protects Boss from being killed and this causes him to go on a domming vacation on Boss.  
-You Need to Stop That - Red/Boss - 307 Words

6\. Classiccherryberry, Undertale or Underswap, Romantic/Sexual, NSFW, Having fun times together ayyy and spitroasting with some fluff.  
-Sweet Thing - Sans/Red/Blue - 531 Words

7\. The weirder the better? In that case I'll venture a request. Characters: how about Fellcest? I like most skeleton ships :3 AU: Underfell in that case. Relationship: Any. SFW/NSFW: either is fine if you can consider it SFW considering the kink. Kink: vore  
-Swallow Your Medicine - Red/Boss - 1,107 Words

8\. For the prompts, can I get a UT Papyrus/UF Papyrus first time having sex? One of them is very nervous and the other encouraging? Possibly with the use of vibrators?

* * *

_**Sort by pairing:** _

2 Papyrus/Sans  
1 Papyrus/Flowey  
1 Papyrus/Edge

1 Sans/Grillby    
1 Sans/Blue  
1 Sans/Blue/Red  -   **[Sweet Thing](http://serifsins.tumblr.com/post/173309426447/classiccherryberry-undertale-or-underswap)**   - 531 words

2 Grillby/Fellby  -   **[Light Me on Fire](http://serifsins.tumblr.com/post/173294552727/fell-grill-by-and-ut-grillby-fell-is-a-huge-flirt)**  and  **[Liquid Flame](http://serifsins.tumblr.com/post/173301187842/fellby-tries-to-convince-grillby-to-let-him-tap) **  - 342 and 340 words  
1 Burgerpants/Blue Bunny  
1 Toriel mastrubation

1 Edge/Fell Flowey  
2 Red/Boss -  **[You Need to Stop That](http://serifsins.tumblr.com/post/173302052862/summary-red-protects-boss-from-being-killed-and)**  and  **[Swallow Your Medicine](http://serifsins.tumblr.com/post/173315450092/the-weirder-the-better-in-that-case-ill-venture) **  - 307 and  _1,107_  words  
1 Red/Tale Flowey  
1 Red/Blue

1 Blue/Stretch/Blue  -  [ **Give the World**](http://serifsins.tumblr.com/post/173295731227/older-brothers-underswap-sans-and-his-big-twin)  723 words

1 G Sans/G Pap (Can someone give me their nicknames? G and ???)  
1 ?Sans and ?Papyrus (My choice, apparently?)

I hope you enjoy!!  
(...my favorite chapter so far is chapter 7, swallow your medicine ;)

**Notes for the Chapter:**

> All of these are available at my blog, SerifSins, where I am quite happy to take in any asks! I don't normally get very many, thus this completely random writing event where I asked everybody and anybody to send in prompts. ^^;; I hope you enjoy reading!


	2. Light Me on Fire

**Summary for the Chapter:**

> "Fell grill by and ut grillby. Fell is a huge flirt to UT. Setting and details : the surface... That's it." -Anonymous

Grillby narrowed his eyes. _Don't you have your own business to be running?_ he crackled near inaudibly, incomprehensible to most of his patrons. The bird on a nearby stool, his self-appointed translator, caught at least half of his words and meanings, and snickered. However, there was one person here that did completely understand the flame tongue, and he wasn’t talking about any skeletons.

No, the purple flame across from him was the one to understand, and Grillby couldn’t help but regret this fact. It had been pleasant at first, but then he realized that the other used their shared language for only one purpose: to flirt with him.

 _Don’t worry, hot stuff_ , Fellby said with the shade of fire that meant he was smirking, _they all know I have something important to tend to. And by tend to, I mean that I want to bend you over and make you hotter than ever._ Grillby flushed, feeling his face flare hotter and just knew that his cheeks were turning blue. His darker counterpart was using his weakness against him – the other knew the pleasure only another flame could give, that everyone else seemed cold in comparison.

Heat, rather than the chill of the atmosphere, traced over Grillby’s hand. He closed his eyes so he could pretend that he didn’t know what was happening, that Fellby wasn’t stroking his hand on the countertop. _Don’t you know that you light me on fire?_ Now Grillby’s entire face felt blue – that was something powerful to say to another flame, and spoke of more than just physical attraction. _You make me burn so bright, I’m going to light you up tonight. I wanna see those pretty flames of yours sparking with my blue, see some purple up inside you~_

Fellby trailed off with a singsong tease, and now Grillby’s facee was blue because there was a burning hand holding his face steady. _C’mon, Grill-baby, let me take care of you tonight._ When Grillby finally opened his eyes, all he could see was blue and purple and _desire_.

**Notes for the Chapter:**

> Continued in Chapter 4: Liquid Fire!


	3. Give the World

**Summary for the Chapter:**

> "Older brothers underswap sans and his big twin / underswap paps. The big bros are protective of papyrus and realize they maybe are overprotective for a bad reason~...." -Anonymous

**Notes for the Chapter:**

> Words: 723  
> Swap Sans has a twin - Comic!

Sans and his identical twin brother, Comic, were working together in the kitchen, both trying their best to prepare a tasty and filling dish for their younger brother. Papyrus had been having a rough day, and they wanted to do anything they could to make it better. The skeleton in question was slumped on the couch in front of the TV, orange-hoodie clad arm draped over his face so that he was blocking out the entire world. Sans felt anger flare in his soul at the thought of how cruel the world could be. How could anyone want to hurt his precious younger brother? He deserved the entire world.

Comic hummed in agreement as he mixed in the spices for the taco, on the same mental wavelength as always. They shared a special bond, one more intimate yet less close than they did with their non-identical sibling. They could practically read each other’s thoughts, more one mind in two bodies than anything else. Which is why, when Comic froze in his motions, Sans instantly had the earth-shattering revelation rock through him as well.

They loved Papyrus – that was nothing new. Papyrus was their whole world, everything they did centered around him. But... that love... their protectiveness... it didn’t stem from purely familial forms. No, they loved him. They would do anything and everything for him. Everything.

Comic met Sans eyes, a new fire lighting them from within. This changed things, yet nothing at all. They had been this way all along, only now – they knew. They knew, and they could act.

* * *

It was several weeks later, and Papyrus was having yet another bad day. Once again, he was curled up on the couch, listening to his two older brothers cook supper. He knew without checking that it would be his favorite meal, that there would be a fresh bottle of honey beside his plate, and a treat of honey lollipops waiting on his bed. Heh. His brothers were the absolute best.

Papyrus shivered as he followed that thought down a widening road, something that he’d always known but had become more obvious in the last few weeks. He loved his brothers so much, looked up to them, admired them, wanted to be with them forever... Wanted to belong to them, wanted them to fuck him, wanted them to use him and cuddle him and take care of him... and he wondered if they wanted the same thing.

It was nothing overt, but his brother’s attention had seen a sudden uptick recently. He had always been the center of their world – something that always made him blush in happiness and embarrassment whenever he thought about it – but it was as if a whole new way to take care of him had opened up. They were constantly brushing up against him, wrapping him up in hugs, grabbing onto his arm and leaning into him, flopping down in his lap and forcing him to pay attention to them, holding onto him...

His face flushed just remembering when they’d started kissing him. It wasn’t really a big deal – Comic had just brushed his teeth against the top of his skull, telling him that he needed to go to bed soon, but he just- Sans did the same thing five minutes later, and then bodily dragged him up the stairs, shoving him down into bed before fussing with the blankets. Comic came in as well, and Papyrus confusedly enjoyed several more hugs and kisses goodnight. It was nice, something they’d once done as young children, but this felt... different. More intense, less innocent.

That was only the first time, and definitely not the only occurrence. Kisses dropped on his skull whenever one of the two wandered by his couch, a peck on the cheek in farewell whenever one of the three had to leave, full body hugs that wrapped him up and made him feel safe...

Papyrus gulped, realizing that he could see Sans in the kitchen. That wasn’t his normal outfit. This new outfit had no right to look that enticing on his older brother. This was so unfair. Comic crossed the kitchen, and Papyrus’ eyes popped when he saw the even more enticing outfit his other older brother was wearing.

Maybe this was just a phase. Yeah, that’s what this had to be.

A phase.


	4. Chapter 4

**Summary for the Chapter:**

> "Fellby tries to convince grillby to let him tap that... By being lewd and romantic. Will he succeed?? And he manages to get his patrons to help too?" -Anonymous

**Notes for the Chapter:**

> Continued from Chapter 2 - Light me on Fire  
> Words: 340

_C’mon, Grill-baby,_ Fellby whispered into his ear. _You know I’d treat you right, feel good inside you._ Grillby didn’t know, but he could guess, and that was enough to make the blush on his face permanent. Even so, he turned away from Fellby, presumably to serve a customer.

Only, when Grillby was about to give the drink to his patron, the fish monster pushed it away. “I know there’ll be plenty of fish in the sea for me, but I don’t know how many fires in the snow there are. Give the drink to the guy, on my tab.” Grillby looked down at the drink, then back at the fish, who went back to looking longingly at his phone.

Arms wrapped around him, one hand folding over his chest and locking him into place, the other reaching for the drink. The sheer heat behind him, engulfing him, let Grillby know exactly who was now well into his personal space. Grillby froze, flames flickering in confusion over what to do but otherwise unmoving, as the hand lifted out of his sight and he could hear Fellby taking several long gulps. The fire around him grew hotter, and began to press into his body, pushing into his back more firmly, flush against his rear.

The hand reentered his view, held up to his mouth. _Drink up_ , said the irritating seducer. _It’ll make you feel good, though not as good as I could._ Grillby parted his lips as the glass was pressed against them, tipping his head back to accept the drink until he could see the flames above. The wicked grin was nothing less than lecherous, and Grillby let a small smile crease his lips.

Grillby slowly swallowed, feeling the light burn of liquor slide down his throat. He delicately used a bright finger to wipe a drop off his lips. He lifted his hand, painting the flaming lips above with the wetness.   _Don’t play with fire unless you’re ready to get burnt,_ he warned. He was about done with playing nice.

**Notes for the Chapter:**

> Bet you weren't ever expecting that patron to show up! XD  
> Also, I have a really hard time flirting without snarking back IRL, because I'm not interested in finding someone and it's often a joke on the flirter's part, so that was hard to resist doing. XD If I ever write a part three for this, you can bet your beer that Grillby is about to be a real spitfire and snark back at Fellby! XD


	5. You Need to Stop That

**Summary for the Chapter:**

> "Summary: Red protects Boss from being killed and this causes him to go on a domming vacation on Boss. Setting is obvious but can be a twist too. Like the surface or swap fell." - Anonymous

**Notes for the Chapter:**

> Words: 307  
> Red is so stressed out by his brother's antics ^^;

“dammit, Boss! you can’t keep doin’ this!” Sans growled, warning in every word. He was maybe being a bit harsh, but Papyrus had nearly gotten himself killed. _Again._

“NYEH-HEH-HEH!” Papyrus laughed, despite the obvious pain it caused him as it jostled his broken ribs. “THIS IS NOTHING TO THE GREAT AND TERRIBLE PAPYRUS!” Sans eye lit red with fury as he stared down at his willful brother.

“Boss. You need to stop.” Sans said quietly, anger growing silent but no less serious. “If you keep this up, you're going to die.” Papyrus winced slightly but stayed firm in staring belligerently up at Sans. “Papyrus,” Sans growled. Papyrus didn’t budge, but neither did Sans. “ _Papyrus._ ”

“NO!” Papyrus blurted out, eyes widening but mouth still running away from him. “I WILL NOT STOP, SANS! I NEED TO KEEP GOING, TO BE STRONGER-“

_ping_

Papyrus slammed into the wall, and Sans had his hand around his throat a moment later. “You. Will. Stop. Doing. This.” He used his magic to draw Papyrus forward slightly, then shoved him back into the wall. “You will _stop_ searching out fights with the most powerful monsters around. ”  Papyrus just looked back at him, unafraid. His arms were folded across his chest, even as his feet hovered above the ground. He knew that Sans would never hurt him, no matter how angry either of them became.

Sans snarled, and Papyrus finally gave up. Sans felt his brother go limp, arms unfolding and coming to rest back at his sides. Papyrus’ pointed head tipped back until it rested against the wall, throat bared under Sans hand. Good. Sans was the one laying down the rules here; Boss could have his control back once he learned to stop risking his life.

Sans fangs glistened in his sharp grin; they would be here for quite a while.

**Notes for the Chapter:**

> Chapter title was originally going to be, You Need to Stop, but that made it sound less consensual than this all really is.


	6. Sweet Thing

**Summary for the Chapter:**

> "Classiccherryberry, Undertale or Underswap, Romantic/Sexual, NSFW, Having fun times together ayyy and spitroasting with some fluff" - Anonymous

**Notes for the Chapter:**

> Words: 531  
> Comic = Undertale Sans

Comic closed his eyes, leaning back and enjoying the sensations. He savored the feelings for a long moment, before opening his eyes and looking back down. He smoothed a hand over the skull before him, red eyes peering up at him the best they could, mouth locked over Comic’s cock. He could hear the light thwacking of Blue’s pelvis smacking into Red’s rear, pausing frequently to grind in as far as it could reach inside Red.

Red was whining pitifully around his cock, and Comic could feel his cock twitch at the sound inside the other’s mouth, warm and wet and partially down the other’s throat. Comic’s conjured cock was long, going all the way through the rougher skeleton’s mouth – he could feel where the other’s tongue ended and his throat began, invisible ecto-flesh wrapping around him and sucking him tight. The change in sensations, the way his cock head popped in and out of Red’s throat as he bobbed his head... his fell counterpart was utterly brilliant at blowjobs.

Judging by Blue’s gasps, he had a really great ass, too. Red was on hands and knees, bottom shoved up in the air for Blue to get to easily, braced well enough that Blue could pump his hips hard without worrying about knocking the small skeleton over. Sans cupped his hands under Red’s chins, cradling his face as he moved back and forth. It had been surprising at first, realizing that Red was the smallest of the three of them, but not so much once you realized how hard it was to acquire resources in his world.

“such a good boy,” Comic murmured, petting the other skeleton and wiping the sweat off his forehead. “such a sweet thing, pleasurin’ us like this. isn’t that right, blue?”

Blue took another moment to push in as far as he could, groaning in synch with Red. As he began to speak, he swiveled his hips slowly, driving in as far as he could go – and by the blue bulge Comic could see if he tilted his head just right, that was pretty darn far. “Mweh- yes, Red, you’re so good!” Blue punctuated with a burst of magic, and Comic watched the bulge grow even bigger as Red froze. “Such a good boy for me, for our friend... You’re so wonderful, ah-“ Blue squeezed his eyes shut as Red clenched down, and Classic was grateful that Red had created a barrier between those sharp teeth and his cock.

 Red was swallowing constantly now, sweet pressure massaging Classic’s cock. Tears were starting to well up in his eyes, and Classic knew that Red was down deep, if emotions were effecting him this powerfully. He wiped away the tears, wishing he could kiss them away but unable to bend that far. Instead, Classic leaned forward, leaning down over Red and trapping him in a hug that Blue echoed moments later.

Red’s limbs gave out, and Sans just managed to support him with blue magic in time to save them all from a tumble. “atta boy,” he murmured, “just let it go and be a good boy for us, just like you’re doin’ now. such a sweet thing.”

**Notes for the Chapter:**

> Hmm... out of all of these, this one might actually fit into my theoretical one big universe connecting most of my tumblr posts!


	7. Swallow Your Medicine

**Summary for the Chapter:**

> "The weirder the better? In that case I'll venture a request. Characters: how about Fellcest? I like most skeleton ships :3 AU: Underfell in that case. Relationship: Any. SFW/NSFW: either is fine if you can consider it SFW considering the kink. Kink: vore :3 [...]" - Lycovore

**Notes for the Chapter:**

> Words: 1,107  
> I didn't expect to, but I enjoyed writing this very much~  
> Nonfatal, harmless, clean vore.

Papyrus watched Sans with a calculating eye, _check_ ing his status and his magic levels. Partially due to childhood malnutrition, the fanged skeleton occasionally had difficulties maintaining his magic levels. After intense expenditure, Sans would become lethargic and have difficulties recovering. Extended bedrest would theoretically be enough to restore his energy, as monsters could absorb ambient magic as well as expel it, should it be necessary. However, in these states, it often took far to long to be safe in their less than ideal world, and Sans system needed a kickstart.

This had been happening for years, and after the first few terrifying times where Papyrus tried his best to keep them both alive rather than the other way around, they had finally come up with a solution. Or rather, Papyrus had, as Sans would never even have considered the possibility.

Papyrus would simply bleed off his excess magic into Sans. Nowadays, he knew that he could channel magic directly into Sans soul, but his childhood method worked much better for Sans’ condition. ...He would also be lying if he said he didn’t like the process. Maybe he enjoyed the entire thing a little too much, but Sans seemed glad that Papyrus actually _enjoyed_ the process rather than it being an imposition, so...

Examining Sans once more, Papyrus prepared for the procedure. It wasn’t enough to just channel magic into the air around Sans – no, Papyrus needed to be _inside_ Sans to do the most good. To fit inside Sans, Papyrus needed to be smaller, to condense his energy, to reduce his form to – at maximum – a fifth of his normal size.

Sans blinked blearily up at Papyrus as he straddled Sans on the bed, black body armor already discarded nearby on the floor. Papyrus propped himself up with his hands against the sofa below, so that he could lean over his much smaller brother. He closed his eyes and began focusing his magic, focusing on the skill he had honed through the years of repeated efforts and attempts.

He shivered as a ripple ran through his body, automatically clenching and fisting his hands at the bizarre sensation of folding in on himself. His soul started to glow faintly as the tremors wracked his form, the magic that his body was truly composed of – not calcium or milk – condensing and withdrawing into itself. He drew his knees forward so that he was once more above Sans stomach, leaning towards Sans face so that he’d fall closer to his goal rather than farther away. The distance between his face and Sans’ seemed to multiply as he shrank, becoming less and less yet still the same skeleton inside.

His soul was burning brightly now, holding the majority of his pent-up self. He didn’t know exactly how small he was, but he knew that he could be smaller, could be easier for Sans to take. So he folded in on himself more, and more, until his energy was straining at the seams and he could barely hold it in. He looked up with a relieved gasp as he locked himself into this form, into this size, into a more morsel sized piece. He saw Sans skull, his eternally grinning canines, and used his brother as a judge for how small he was.

Papyrus was now approximately the size of his brother’s fist – not the biggest meal his brother had ever swallowed whole, but still large enough to be a bit awkward going down. As a weak hand landed beside him to help take him to Sans’ mouth, he realized that he was slightly larger than he had guessed... but Sans could still take him in. His jaw might be sore the next day, but they could still do this easily enough. He held tight to one of Sans fingers as the hand lifted him away from his brother’s chest, practice helping him stay steady as Sans sat upright on the bed.

The gentle hand slowly tipped, letting Papyrus step off onto Sans upturned face. No talking was necessary – they were both ready for this. Sans jaw shifted, opening as far as it could – and then wider still. He looked like one of those human sword swallowers, head tilted back as far as it could go, a nearly straight line from mouth to stomach. There was a faint glow of red beneath his chin as his ecto body shimmered into visibility, preparing for extra strain.

Papyrus stepped into Sans mouth. He stopped when he heard Sans moan, arms using Sans teeth as a stop and his body lying flat against Sans thick tongue. Papyrus began to sweat in reaction to the tight heat, the wetness already beginning to encapsulate his body. Normally he would be disgusted at the thought of being covered with saliva... but as it was, he simply shivered and began to shimmy backwards. His feet dangled out the back of Sans mouth, down into his throat as the rest of Papyrus’ body was slowly swallowed up. He lowered himself more and more, feeling Sans spasmodically swallowing, pulling Papyrus down further into his gullet. Over half of Papyrus was down Sans’ throat now, only the top part of his body in Sans mouth, and everything went red. Sans had closed his mouth, and now only his brother’s magic lit Papyrus’ surroundings.

Well, Papyrus’ own body was giving off it’s own red glow as well – in fact, as Papyrus head was squeezed within Sans ethereal trachea, his body was glowing brighter than it ever did at full size. The amount of magic dissipating off of him was enormous, especially in comparison to his tiny size. Then again, that was exactly why his magic was leaking out – it didn’t all fit in his small frame, and it had to escape somewhere. That somewhere was straight into Sans, where it became a part of his brother’s magic and strengthened his body and soul.

Papyrus moaned as he settled inside Sans stomach, curled up into fetal position as he rocked inside his brother. He didn’t know how long he was lost in sensation, in feeling safe and warm and held tight inside his precious sibling. Papyrus just knew that everything was _good._

Entirely out of habit, Papyrus _check_ ed his brother, and was relieved to find that Sans magic levels had basically doubled from what they had been earlier. If that was the case, Sans would probably begin moving around soon. His brother had an appearance to put in at Grillby’s, and the sooner he showed his strength, the better. Nobody knew about Sans’ weakness yet – and if Papyrus had his way, nobody ever would.

**Notes for the Chapter:**

> I actually really want to keep writing this someday... I have a few ideas:  
> -How Papyrus is hidden inside Sans as his brother goes about his daily life  
> -That he lets himself grow a little bigger now that he’s been succesfully swallowed  
> -How Sans pretends Pap is on vacation - and honestly? To Papyrus, this is a vacation. He doesn’t have to do anything, and he is intimately aware of his brother and their safety.


End file.
